Abstract
Ambient noise correlation is capable of retrieving waves propagating between two receivers. Although waves retrieved using this technique are primarily surface waves, the retrieval of body waves, including direct, refracted, and reflected waves, has also been reported from land-based observations. The difficulty of body wave extraction may be caused by large amplitudes and little attenuation of surface waves excited by microseisms, indicating that body wave extraction using seafloor records is very challenging because microseisms are generated in ocean areas and large amplitudes of surface waves are presumably observed at the seafloor. In this study, we used a unique dataset acquired by dense arrays deployed in the Nankai subduction zone, including a permanent cabled-network of 49 stations, a borehole sensor, and 150 temporary stations, to attempt to extract near-field body waves from ambient seafloor noise observed by multivariate sensors of broadband and short-period seismometers, differential pressure gauges (DPGs), and hydrophones. Our results show thatPwaves are extracted only in the DPG-record correlations at a frequency of 0.2–0.5 Hz, which can be seen up to a separation distance of two stations of 17 km with an apparent velocity of 3.2 km/s. At 1–3 Hz,Pwaves are observed only in the vertical-record correlations up to a separation distance of 11 km with an apparent velocity of 2.0 km/s. These velocity differences reflect the vertical velocity gradient of the accretionary prism, because thePwaves at low frequencies propagate at relatively long distances and therefore the turning depth is greater. Moreover, the long-period and short-periodPwaves are observed at the slope and flat regions on the accretionary prism, respectively. To investigate the retrieved wavefield characteristics, we conducted a two-dimensional numerical simulation for wave propagations, where we located single sources at the sea surface above the flat and slope bathymetry regions. Based on our observations and simulations, we suggest that the retrieval of near-field body waves from ambient seafloor noises depends on the relative amplitudes ofPand other surface waves in the ambient noise wavefield, and those are controlled by the subseafloor velocity structure, seafloor topography, and water depth.
Highlights
Ambient noise analysis applied to land-based seismic records has retrieved various wavefields propagating between two receivers (Wapenaar, 2004)
The difficulty of extracting body waves is primarily caused by large amplitudes of surface waves observed at land stations, which correspond to microseisms excited by ocean swells in the ocean areas (Longuet-Higgins, 1950; Hasselmann, 1963)
P wave propagations are observed up to a separation distance of 17 km in the positive lag time along the travel time curve of the P waves estimated from the velocity structure (Tonegawa et al, FIGURE 2 | Waveform examples recorded with borehole and seafloor sensors. (A) One-hour records of the vertical component at 1–3 Hz observed at the borehole sensor on February 9, 2016. (B) Same as (A), but for KMD16. (C) One-hour Cross-correlation functions (CCFs) using the waveforms in (A,B), with the borehole as the reference site
Summary
Ambient noise analysis applied to land-based seismic records has retrieved various wavefields propagating between two receivers (Wapenaar, 2004). Teleseismic body waves excited in the ocean areas can be observed at land stations (Gerstoft et al, 2006; Koper et al, 2010; Landès et al, 2010; Gualtieri et al, 2014; Farra et al, 2016; Nishida and Takagi, 2016). This means that most of the body wave energy is transmitted to the interior of the Earth and can be observed at distant stations, near-field body waves are possibly observed under the conditions of dense seismic sensors deployed at the seafloor
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